U.S. patent application number 11/747701 was filed with the patent office on 2008-07-24 for solid state lighting devices comprising quantum dots.
This patent application is currently assigned to EVIDENT TECHNOLOGIES, INC.. Invention is credited to Kwang-Ohk CHEON, David DUNCAN, Jennifer GILLIES, Michael LoCASIO, David SOCHA.
Application Number | 20080173886 11/747701 |
Document ID | / |
Family ID | 39640365 |
Filed Date | 2008-07-24 |
United States Patent
Application |
20080173886 |
Kind Code |
A1 |
CHEON; Kwang-Ohk ; et
al. |
July 24, 2008 |
SOLID STATE LIGHTING DEVICES COMPRISING QUANTUM DOTS
Abstract
Solid state lighting devices containing quantum dots dispersed
in polymeric or silicone acrylates and deposited over a light
source. Solid state lighting devices with different populations of
quantum dots either dispersed in matrix materials or not are also
provided. Also provided are solid state lighting devices with
non-absorbing light scattering dielectric particles dispersed in a
matrix material containing quantum dots and deposited over a light
source. Methods of manufacturing solid state lighting devices
containing quantum dots are also provided.
Inventors: |
CHEON; Kwang-Ohk; (Latham,
NY) ; GILLIES; Jennifer; (Petersburg, NY) ;
SOCHA; David; (Delmar, NY) ; DUNCAN; David;
(Troy, NY) ; LoCASIO; Michael; (Clifton Park,
NY) |
Correspondence
Address: |
KENYON & KENYON LLP
1500 K STREET N.W., SUITE 700
WASHINGTON
DC
20005
US
|
Assignee: |
EVIDENT TECHNOLOGIES, INC.
Troy
NY
|
Family ID: |
39640365 |
Appl. No.: |
11/747701 |
Filed: |
May 11, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60799311 |
May 11, 2006 |
|
|
|
Current U.S.
Class: |
257/98 ;
257/E21.002; 257/E33.068; 313/498; 438/29 |
Current CPC
Class: |
H01L 33/501 20130101;
C09K 11/7774 20130101; C09K 11/02 20130101; H01L 33/502 20130101;
C09K 11/883 20130101 |
Class at
Publication: |
257/98 ; 313/498;
438/29; 257/E21.002; 257/E33.068 |
International
Class: |
H01J 1/62 20060101
H01J001/62; H01L 33/00 20060101 H01L033/00; H01L 21/02 20060101
H01L021/02 |
Claims
1 A solid state lighting device comprising: a light source; and an
active layer deposited either directly or indirectly on top of the
light source, the active layer comprising one or more populations
of quantum dots dispersed in a first matrix material, wherein the
first matrix material comprises a polymer or silicone having a
plurality of cross-linked acrylate groups.
2. The device of claim 1, wherein the first matrix material is a
urethane acrylate, polyacrylate, acrylated silicone, urethane
acrylate epoxy mixture, or a combination thereof.
3. The device of claim 1, further comprising: an encapsulant layer
deposited on top of the light source, the encapsulant layer
comprising a second matrix material, and the active layer deposited
on top of the encapsulant layer.
4. The device of claim 3, wherein the second matrix material
comprises a polyacrylate, acrylated silicone, urethane acrylate,
epoxy, silicone, sol-gel, nanoclay, or a combination thereof.
5. The device of claim 1, further comprising another encapsulant
layer deposited on top of the active layer, the another encapsulant
layer comprising a third matrix material.
6. The device of claim 3, further comprising another encapsulant
layer deposited on top of the active layer, the another encapsulant
layer comprising a third matrix material.
7. The device of claim 5, wherein the third matrix material
comprises a polyacrylate, acrylated silicone, urethane acrylate,
epoxy, silicone, sol-gel, nanoclay, or a combination thereof.
8. The device of claim 1, wherein the active layer comprises a
composite comprising micronized quantum dot complexes dispersed in
the first matrix material, the micronized quantum dot complexes
comprising the one or more populations of quantum dots dispersed in
a base material, wherein the base material is micronized.
9. The device of claim 1, wherein the light source is a light
emitting diode chip, a laser, a white light, a lamp, or any
suitable combination thereof.
10. The device of claim 1, wherein the solid state lighting device
is a light emitting diode.
11. The device of claim 1, wherein the solid state lighting device
is a laser diode.
12 The device of claim 1, further comprising a reflector cup
housing the light source.
13. A method of manufacturing a solid state lighting device
comprising: providing a light source; dispersing quantum dots in a
polymer or silicone having acrylate groups to form a first matrix
material; depositing the first matrix material either directly or
indirectly on top of the light source; and cross-linking the
acrylate groups in the first matrix material to form a solid active
layer.
14. The method of claim 13, wherein the quantum dots are dispersed
in the polymer or silicone without a solvent.
15. The method of claim 13, wherein depositing a first matrix
material either directly or indirectly on top of the light source
comprises: depositing an encapsulant layer on top the light source,
the encapsulant layer comprising a second matrix material; and
depositing the first matrix material on top of the encapsulant
layer.
16. The method of claim 13, wherein cross-linking the acrylate
groups comprises exposing the first matrix material to ultraviolet
irradiation.
17. The method of claim 16, wherein a curing dosage of the
ultraviolet irradiation is 0.6 J/cm.sup.2.
18. The method of claim 13, wherein cross-linking the acrylate
groups comprises: adding a thermal or an ultraviolet initiator to
the first matrix material; and applying heat or ultraviolet
irradiation to the first matrix material to cross-link the acrylate
groups.
19. The method of claim 18, wherein the thermal initiator is
azobisisobutyronitrile.
20. The method of claim 18, wherein the ultraviolet initiator is 1
hydroxy cyclohexyl phenyl ketone.
21. The method of claim 13, wherein cross-linking the acrylate
groups comprises: adding a chemical additive to the first matrix
material to cross-link the acrylate groups.
22. The method of claim 21, wherein the chemical additive is an
amine, diamine, oleyl amine, docecyl amine,
aminopropylmethoxysilane, bis(3-aminopropyl)-tetramethyl
disiloxane, 3-aminopropyl dimethyl ethoxysilane,
3-aminopropylmethyl bis-(trimethyl siloxy) silane, or a combination
thereof.
23. The method of claim 13, wherein the acrylate groups are
cross-linked via an ultraviolet or thermal initiated vinylic
addition, Michael addition or condensation.
24. The method of claim 13, further comprising: adding 20% methyl
hexahydrophthalic anhydride to the first matrix material.
25. The method of claim 13, further comprising: adding an epoxy to
the first matrix material.
26. A solid state lighting device comprising: a light source; and
an active layer deposited either directly or indirectly on the
light source, the active layer comprising: a first matrix material;
a population of quantum dots dispersed in the first matrix
material; and non-absorbing light scattering dielectric particles
dispersed in the first matrix material, wherein the particles have
a diameter between 2 nanometers and 50 microns, have refractive
indices greater than that of the first matrix material, and are
substantially non-absorbent to light emitted by the light source or
the population of quantum dots.
27. The device of claim 26, wherein the non-absorbing light
scattering dielectric particles comprise titania or alumina
particles.
28. The device of claim 26, wherein the concentration of the
non-absorbing light scattering dielectric particles in the first
matrix material is 0.1%-20%.
29. The device of claim 26, further comprising a transparent lens
cap overcoating the active layer either directly or indirectly.
30. A solid state lighting device comprising: a light source; a
first active layer deposited either directly or indirectly on top
of the light source, the first active layer comprising a first
population of quantum dots; and a second active layer deposited
either directly or indirectly on top of the first active layer, the
second active layer comprising a second population of different
quantum dots.
31. The solid state lighting device of claim 30, further
comprising: an encapsulant layer deposited on top of the light
source, the encapsulant layer comprising a second matrix material,
and the first active layer deposited on top of the encapsulant
layer.
32. The device of claim 30, further comprising another encapsulant
layer deposited on top of the second active layer, the another
encapsulant layer comprising a third matrix material.
33. The device of claim 30, wherein the first population of quantum
dots is dispersed in a first matrix material and the second
population of different quantum dots is not dispersed in a matrix
material.
34. The device of claim 30, wherein a spacer film is disposed
between the first and second active layers.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to U.S. Provisional
Application Ser. No. 60/799,311, filed on May 11, 2006, which is
incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present invention relates to solid state lighting
devices comprising quantum dots. The present invention also relates
to methods of making solid state lighting devices comprising
quantum dots.
BACKGROUND OF THE INVENTION
[0003] Light emitting diodes (LEDs) are solid state semiconductor
devices that emit light with a narrow spectral distribution when an
electric current is applied. The wavelength of light emitted by the
LEDs is a direct result of the bandgap of the emissive layer
comprising the quantum dot which is, in turn, related to the
semiconductor composition.
[0004] High brightness blue (peak wavelength between 450 nm and 470
nm), violet (peak wavelength .about.410 nm) and ultraviolet LEDs
(peak wavelength .about.380 nm) have improved in terms of both
brightness, efficiency, and longevity. Green indium gallium nitride
(InGaN) LEDs (peak wavelength .about.520 nm) are also available,
however, the efficiency of LEDs made of this material system drops
precipitously for wavelengths approaching 555 nm green.
[0005] The first and most common method to achieve white light from
an LED is to combine a phosphor powder with an epoxy or silicone
encapsulant and apply the mixture onto the surface of an InGaN LED
chip or within a reflector cup containing a blue InGaN LED chip.
The phosphor absorbs a portion of the blue light emitted by the
underlying LED chip and down converts that light to a slightly
longer broadband yellow wavelength. At the appropriate phosphor
combination, the ratio of broadband yellow light plus the residual
blue light derived from the LED chip that is not absorbed by the
phosphor yields a white color. See Schotter P., "Luminescence
Conversion of Blue Light Emitting Diodes," App. Phys. A., Vol. 64,
pgs. 417-418 (1997). Similarly, other specialty colors such as pink
can be made by adding "red" emitting phosphors to a blue emitting
LED chip. Lanthanide doped garnets, nitrides and orthosilicates are
the most widely used types of phosphors for LED application.
Exemplary broadband yellow phosphors used to create white light
include cerium doped yttrium aluminum garnet (Ce:YAG) or cerium
doped terbium aluminum garnet (Ce:TAG). A typical emission spectrum
of the white light LEDs, prepared by combining the YAG phosphor
with a blue light, has two distinct peaks, where the first peak
corresponds to blue LED emission, .about.470 nm, and the second
peak corresponds to the emission of the YAG phosphor, .about.555
nm. Generally speaking, white light made in this way is of poor
color quality (low color rendering index--CRI) and can reach a
limited range of white color temperatures (typically 6500-4500K).
Phosphors generally have a fairly narrow absorption spectra and as
such can only be used on underlying light sources having a very
specific range of emission wavelengths. The Ce:YAG is optimized for
460 nm light but is poorly suited for LED chips emitting at any
other wavelength.
[0006] High brightness LEDs including white and specialty color
LEDs have diverse applications including traffic signals, signage
and display lighting, architectural lighting, LCD display
backlights used in mobile phones and PDAs, larger flat panel LCD
backlights and projectors/projection TV, outdoor/landscape lighting
luminaires, interior illumination in the transportation sector
(airplanes, subways, ships, etc.), and automobiles. As such there
is a need for bright long lasting LEDs available in a wide variety
of colors.
[0007] Quantum dots (also known as semiconductor nanocrystals) can
be used as down converters applied onto short wavelength LED chips
and used to generate the visible and infrared light. Quantum dots
are tiny crystals of II-VI, III-V, IV-VI materials that have a
diameter between 1 nanometer (nm) and 20 nm. In the strong
confinement limit, the physical diameter of the quantum dot is
smaller than the bulk excitation Bohr radius causing quantum
confinement effects to predominate. In this regime, the quantum dot
is a 0-dimensional system that has both quantized density and
energy of electronic states where the energy differences between
electronic states are a function of both the quantum dot
composition and the physical size of the quantum dot itself. Larger
quantum dots have more closely-spaced energy states and smaller
quantum dots have the reverse. Because interaction of light and
matter is determined by the density and energy of electronic
states, many of the optical and electric (optoelectronic)
properties of quantum dots can be tuned or altered simply by
changing the quantum dot geometry (i.e. physical size).
[0008] Single quantum dots or monodisperse populations of quantum
dots exhibit unique optical properties that are size tunable. Both
the onset of absorption and the photoluminescent wavelength are a
function of quantum dot size and composition. The quantum dots will
absorb all wavelengths shorter than the absorption onset, however
photoluminescence will always occur at the absorption onset. The
bandwidth of the photoluminescent spectra is due to both
homogeneous and inhomogeneous broadening mechanisms. Homogeneous
mechanisms include temperature dependent Doppler broadening and
broadening due to the Heisenberg Uncertainty Principle, while
inhomogeneous broadening is due to the size distribution of the
quantum dots. The narrower the size distribution of the quantum
dots, the narrower the full-width half-max (FWHM) of the resultant
photoluminescent spectra. In 1991, Louis Eugene Brus wrote a paper
reviewing the theoretical and experimental research conducted on
colloidally grown quantum dots, such as cadmium selenide (CdSe) in
particular (Brus L., Quantum Crystallites and Nonlinear Optics,
Applied Physics A, 53 (1991)). That research, precipitated in the
early 1980's by the likes of Efros, Ekimov, and Brus himself,
greatly accelerated by the end of that decade as demonstrated by
the increase in the number of papers concerning colloidally grown
quantum dots.
[0009] For a given quantum dot, the emission band is dependant on
the size of the quantum dot. For instance, CdSe covers the whole
visible range: the 2 nm diameter CdSe quantum dot emits in the blue
range and 10 nm CdSe emits in the red range.
[0010] Therefore quantum dots are useful as a novel optical down
converter that, when combined with a light emitting diode light
source, could produce a range of colors that are unattainable with
conventional phosphors. One of the challenges to date, however, is
that quantum dots are susceptible to degradation when dispersed in
many polymeric materials that results in degradation of brightness.
Quantum dots are also susceptible to photo-oxidation which results
in permanent degradation of brightness over time when exposed to
oxygen and light. Furthermore, quantum dot brightness is also
reduced at elevated temperatures such as those found on the
surfaces of LED chips. Lastly, the process by which quantum dots
are applied to LED chips should be compatible with contemporary
manufacturing processes.
[0011] Until now there were several manners in which to apply
quantum dots as down converters. Bawendi et al. has demonstrated
that nanocrystals may be dispersed within polystyrene solution and
applied to the surface of an LED. However, this method requires
that the solvent in which the polystyrene and nanocrystals are
dispersed be evaporated which is incompatible with conventional
manufacturing processes. This may also result in a porous
nanocrystal composite that does not protect the nanocrystals from
oxygen and thus enables photo-oxidative degeneration of the
nanocrystals. Furthermore, polystyrene is subject to degradation
(yellowing) itself under the intense light of an LED chip. Bawendi
et al. also demonstrated that nanocrystals in various solvents may
be added to methacrylate monomers or epoxies which react to for a
polymeric solid. However again, the use of solvents results in
porous films and subject the nanocrystals to photo-oxidative
degradation. Those methods are also incompatible with conventional
LED manufacturing processes. Rohwer et al. demonstrated white light
LEDs comprising a "blue" InGaN LED chip upon which CdS nanocrystals
were dispersed. The CdS nanocrystals were prepared in such a way
that there existed a prevalence of defects on the nanocrystal
surface that result in well known broadband surface trap emission.
This light emission mechanism is inefficient and results in low
efficacy LEDs. See U.S. Pat. No. 6,914,265, U.S. Pat. No.
6,890,777, U.S. Pat. No. 6,803,719, U.S. Pat. No. 6,501,091 and
Rohwer L., "Development of Solid State Lighting Devices Based on
II-VI Semiconductor Quantum Dots," Proc. of the SPIE, Vol. 5366
pages 66-74.
[0012] As such, there is a need in the art for a solid state
lighting devices that do degrade under the intense illumination of
the underlying light source, are compatible with conventional LED
packaging methodologies, do not degrade the brightness of the
quantum dots and/or protect the quantum dots from
photo-oxidation.
SUMMARY OF THE INVENTION
[0013] The present invention provides solid state lighting devices,
methods of making the same, and apparatuses comprising solid state
lighting devices.
[0014] In an embodiment, the present invention provides a solid
state lighting device comprising a light source and an active layer
deposited either directly or indirectly on the light source. The
active layer comprises a population of quantum dots dispersed in a
first matrix material, wherein the first matrix material comprises
a polymer or silicone having a plurality of cross-linked acrylate
groups. In certain embodiments, an encapsulant layer is disposed
between the light source and the active layer. In additional or
alternative embodiments, another encapsulant layer is disposed on
top of the active layer.
[0015] In other embodiments, the present invention provides a solid
state lighting device comprising a light source and an active layer
deposited either directly or indirectly on the light source. The
active layer comprises a first matrix material and a population of
quantum dots dispersed in the first matrix material. The active
layer further comprises non-absorbing light scattering dielectric
particles dispersed in the first matrix material. The particles
have a diameter between about 2 nanometers and 50 microns, have
refractive indices greater than that of the first matrix material,
and are substantially non-absorbent to light emitted by the light
source or the population of quantum dots.
[0016] In other embodiments, the present invention provides a
method of manufacturing a solid state light emitting device
comprising providing a light source, dispersing quantum dots in a
polymer or silicone having acrylate groups to form a first matrix
material and depositing the first matrix material either directly
or indirectly on the light source. The method further comprises
cross-linking the acrylate groups in the first matrix material to
form a solid active layer. The acrylate groups can be cross-linked
by various methods including, for example, by chemical additives,
ultraviolet radiation, an electron beam or heat.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus are
not limitative of the present invention, and wherein:
[0018] FIG. 1 is a schematic illustration of a solid state lighting
device according to an embodiment of the present invention.
[0019] FIG. 1a is a schematic illustration of a solid state
lighting device according to another embodiment of the present
invention.
[0020] FIG. 2 is a schematic illustration of a solid state lighting
device according to another embodiment of the present
invention.
[0021] FIG. 3 is a schematic illustration of a solid state lighting
device according to another embodiment of the present
invention.
[0022] FIG. 4 is a schematic illustration of a solid state lighting
device according to another embodiment of the present invention
[0023] FIG. 5 is a spectral graph for solid state lighting devices
according to embodiments of the present invention illustrating the
relationship between the light source-active layer distance and the
intensity of the light emitted by the device.
[0024] FIG. 6 is a spectral graph for a solid state lighting device
according to an embodiment of the present invention when emitting
green light and having a blue light source.
[0025] FIG. 7 is a spectral graph for a solid state lighting device
according to an embodiment of the present invention when emitting
white light and having a blue light source.
[0026] FIG. 8 is a spectral graph for a solid state lighting device
according to an embodiment of the present invention when emitting
infrared light and having a blue light source.
[0027] FIG. 9 is a spectral graph for a solid state lighting device
according to an embodiment of the present invention when emitting
red and yellow light from multiple active layers and having a blue
light source.
[0028] FIG. 10 is a spectral graph for a solid state lighting
device according to an embodiment of the present invention
illustrating the stability of a white LED monitored for 4000
hours.
[0029] FIG. 11 is a spectral graph for a solid state lighting
device according to an embodiment of the present invention when
emitting red light.
[0030] FIG. 12 is a CIE diagram showing the measured coordinates of
the emitted colors for solid state lighting devices according to
embodiments of the present invention having a green light
source.
[0031] FIG. 13 is a CIE diagram showing the measured coordinates of
the emitted colors for solid state light devices according to
embodiments of the present invention having an ultraviolet light
source.
[0032] FIG. 14 is a spectral graph for a solid state lighting
device according to embodiments of the present invention when
emitting red light and having been thermally cured.
[0033] FIG. 15 is a spectral graph for a solid state lighting
device according to an embodiment of the present invention when
emitting warm white light and having been thermally cured.
DETAILED DESCRIPTION OF THE INVENTION
[0034] In certain embodiments, the present invention provides solid
state lighting devices. Referring to FIG. 1, in certain
embodiments, the present invention provides a solid state lighting
device 10 comprising a light source 20 and an active layer 30
deposited directly on the light source 20. Active layer 30
comprises a first matrix material 35 comprising a polymer or
silicone having a plurality of cross-linked acrylate groups.
Dispersed within first matrix material 35 are one or more
populations of quantum dots 40. The above-mentioned polymers and
silicones have been found to substantially maintain the initial
quantum yield of the quantum dots, degrade very slowly when exposed
to intense light derived from the light source, protect the quantum
dots from photo-oxidation and may be applied to the light source
using conventional solid state lighting device packaging
methodologies. The first matrix material is preferably transparent
to both the wavelength of light emitted by the underlying light
source as well as the light wavelength(s) emitted by each
population of quantum dots dispersed within it. Non-limiting
examples of acrylated polymers and silicones include urethane
acrylate, polyacrylate, acrylated silicone, urethane acrylate epoxy
mixture, or a combination thereof. Particularly preferred acrylated
polymers or silicones are OP-54.TM. (Dymax) and ZIPCONE.TM.
(Gelest).
[0035] Each population of quantum dots dispersed within the first
matrix material absorbs a portion of the light emitted by the
underlying light source and emits light at a longer wavelength,
where the peak emission wavelength of each quantum dot population
is dependent upon the composition and mean diameters of the quantum
dots themselves. Each population of quantum dots is composed of a
plurality of similar quantum dots in both composition and size. The
quantum dots comprise a quantum dot core having an outer surface.
The quantum dot core may be spherical nanoscale crystalline
materials (although oblate and oblique spheroids can be grown as
well as rods and other shapes) having a diameter of less than the
Bohr radius for a given material and typically but not exclusively
comprises II-IV, III-V, and IV-VI binary semiconductors.
Non-limiting examples of the semiconductor materials that the
quantum dot core may comprise include ZnS, ZnSe, ZnTe, CdS, CdSe,
CdTe, HgS, HgSe, HgTe (II-VI materials), PbS, PbSe, PbTe (IV-VI
materials), AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP,
InAs, InSb (III-V materials). In addition to binary semiconductors,
the quantum dot core may comprise ternary, quaternary, or quintary
semiconductor materials. Non-limiting examples of ternary,
quaternary, or quintary semiconductor materials include
A.sub.xB.sub.yC.sub.zD.sub.wE.sub.2v wherein A and/or B may
comprise a group I and/or VII element, and C and D may comprise a
group III, II and/or V element although C and D are not group V
elements, and E may comprise a VI element, and x, y, z, w, and v
are molar fractions between 0 and 1.
[0036] In addition to the quantum dot core having an outer surface,
the quantum dot composition may comprise a shell formed on the
outer surface of the core. The shell is typically, although not
always, between 0.1 nm and 10 nm thick. The shell may provide for a
type A quantum dot composition. Shells may comprise various
different semiconductor materials such as, for example, CdSe, CdS,
CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, InP, InAs, InSb, InN, GaN,
GaP, GaAs, GaSb, PbSe, PbS, and PbTe. The shell may be formed
directly on the outer surface of the core or over one or more
intermediate layers, such as the metal layer described below,
formed on the outer surface of the core.
[0037] In an alternate embodiment, the quantum dot composition may
comprise the quantum dot core having an outer surface and one or
more metal layers formed on the outer surface of the core after
synthesis of the core. The metal layer may act to passivate the
outer surface of the quantum dot core and limit the diffusion rate
of oxygen molecules to the core. The metal layer is typically,
although not always, between 0.1 nm and 5 nm thick. The metal layer
may include any number, type, combination, and arrangement of
metals. For example, the metal layer may be simply a monolayer of
metals formed on the outer surface of the core or multiple layers
of metals formed on the outer surface. The metal layer may also
include different types of metals arranged, for example, in
alternating fashion. Further, the metal layer may encapsulate the
quantum dot core or may be formed on only parts of the outer
surface of the core. The metal layer may include the metal from
which the quantum dot core is made either alone or in addition to
another metal. Non-limiting examples of metals that may be used as
part of the metal layer include Cd, Zn, Hg, Pb, Al, Ga, or In.
[0038] In another alternate embodiment, the quantum dot composition
may comprise the quantum dot core having an outer surface, one or
more metal layers formed on the outer surface of the core after
synthesis of the core, and the shell overcoating the metal
layer(s).
[0039] The quantum dot core, shell, and/or metal layer may be grown
by the pyrolysis of organometallic precursors in a chelating ligand
solution or by an exchange reaction using the prerequisite salts in
a chelating ligand solution. The chelating ligands are typically
lyophilic and have a moiety with an affinity for the outer layer of
the quantum dot composition and another moiety with an affinity
toward the solvent, which is usually hydrophobic. Typical examples
of chelating ligands include lyophilic surfactant molecules such as
Trioctylphosphine oxide (TOPO), Trioctylphosphine (TOP),
Tributylphosphine (TBP), primary amines, and organic acids. The
ligands used throughout the quantum dot synthesis remain on the
surface of the quantum dots after the quantum dots are removed from
the reaction mixture. Thus the ligands used during synthesis
comprise the surfactant layer. Though the surfactant molecules may
include a phosphine moiety, it is to be appreciated that other
chelating ligands may be used.
[0040] The surfactant layer of the quantum dot complex typically
includes organic molecules that have a moiety with an affinity for
the surface of the quantum dot and another hydrophobic moiety
(typically alkane, aromatic or other nonpolar or non-ionizable
moiety, e.g., TOP is terminated with three nonpolar octane groups)
with an affinity for a hydrophobic solvent. Moieties that have an
affinity to the surface of the quantum dot include thiols, amines,
phosphines, and phosphine oxides. Surfactants, such as TOPO, TOP,
and TBP, are typically used in the synthesis of the quantum dots
and can remain on the dot's surface after preparation of the dot or
may be added or replaced by other surfactants after synthesis. The
surfactant layer tends to assemble into a coating around the
quantum dot and enables the dot to suspend in a hydrophobic
solvent.
[0041] In another embodiment of the invention, the active layer of
a solid state light emitting device includes conventional phosphors
that are added with the quantum dots to the first matrix material.
Non-limiting examples of conventional phosphors include cesium
yttrium aluminum garnet, europium yttrium aluminum garnet, europium
orthosilicates, cesium terbium aluminum garnet, and europium
nitrides.
[0042] It is appreciated that although FIG. 1 (and other figures
described below) shows the active layer as one single layer, it may
comprise more than one layer. Further, in certain embodiments, a
population of quantum dots are not dispersed in any matrix material
but rather directly deposited on the underlying layer of light
source. For example, referring to FIG. 1a, in certain embodiments,
a solid state lighting device 10 comprises a first active layer 30a
with a first population of quantum dots and a second active layer
30b with a second population of different quantum dots disposed on
top of first active layer 30a. In certain embodiments, the second
population of different quantum dots are not dispersed in a matrix
material, whereas the first population may be as illustrated in
FIG. 1a. In other embodiments, a spacer film 80 separates the first
and second active layers. The spacer film can comprise any suitable
material such as a polymeric or silicone material. In certain
embodiments, the spacer film is fabricated from polyacrylate.
[0043] Referring to FIG. 2, in other embodiments, the present
invention provides a solid state lighting device 10 comprising a
light source 20 and an active layer 30 deposited directly on light
source 20. Active layer 30 comprises a first matrix material 35 and
a population of quantum dots 40. Active layer 30 further comprises
light scattering dielectric particles 45 dispersed in first matrix
material 35. The particles have a diameter between about 2
nanometers and 50 microns, have refractive indices greater than
that of first matrix material 35, and are substantially
non-absorbent to light emitted by light source 20 or the population
of quantum dots 40. A purpose of these particles is to scatter the
light emitted by the underlying light source as well as the quantum
dots and cause the respective light to mix more evenly. For
example, there may be a non-uniform angular color distribution
where the observer would see more light from the underlying light
source when viewing the solid state lighting device along its axis
and a prevalence of the light emitted by the quantum dots when
viewed from an off angle. For example, a solid state light emitting
device composed of a "blue" emitting LED chip and "red" emitting
quantum dots would appear bluer when viewed directly along the axis
and redder when viewed off axis.
[0044] Non-limiting examples of light scattering dielectric
particles include titania, alumina, and other metal oxides. In a
preferred embodiment, the light scattering dielectric particles are
titania particles with an average diameter of 5 nm, loaded at 1% by
weight. Larger particles can be used as well, with some adjustment
to the loading. These materials can be dispersed directly into the
first matrix material prior to curing.
[0045] In another embodiment of the present invention, the active
layer comprises a first matrix material and plurality of
microparticles dispersed in the first matrix material. The
microparticles further comprise one or more populations of quantum
dots dispersed within a second transparent matrix material that has
micron scale dimensions. Said second matrix material may be
composed of sol-gel, polymers, silicones, polyurathatne acryalate,
and other materials that do not degrade the luminosity of the
quantum dots. The active layer may further comprise light
scattering dielectric particles dispersed in the first matrix
material. The dielectric particles may have average diameters
ranging from 100 nm to 50 microns. A wide range of first matrix
materials can be used, including sol-gel glasses, glass, polymers,
and epoxy. In one embodiment, the quantum dot microparticles may be
formed by dispersing the quantum dots into a suitable first matrix
material, curing the material, and then milling the cured material
to 0.20 microns to form the microparticles. In another embodiment,
the quantum dot microparticles may be formed by absorbing the
quantum dots onto the surface of fumed silica and then powdering
the resulting quantum dot-silica composite to form the
microparticles. The microparticles may then be mixed directly into
an appropriate first matrix material, along with the dielectric
particles.
[0046] Referring to FIG. 3, in certain embodiments, a solid state
lighting device 10 as described above or according to other
embodiments of the present invention has an encapsulant layer 50
between active layer 30 and light source 20. Encapsulant layer 50
comprises a second matrix material 51, non-limiting examples of
which are silicones, epoxies, acrylates, plastics and combinations
thereof including a polyacrylate, an acrylated silicone,
polyurethane acrylate, epoxy, silicone, sol-gel, nanoclay, or a
combination thereof. Encapsulant layer 50 can prevent excessive
heat generated by light source 20 from reaching the population of
quantum dots, which could cause heat degradation. The second matrix
material can be deposited on the light source and then cured by,
for example, ultraviolet (UV) or thermal curing, solvent
evaporation, or some other chemical reaction, such as a Michael
reaction.
[0047] Alternatively or in addition, referring to FIG. 4, in
certain embodiments, a solid state lighting device 10 comprises
another encapsulant layer 60 disposed on top of active layer 30.
Another encapsulant layer 60 comprises a third matrix material 61
which can include the same non-limiting examples as described above
with respect to encapsulant layer 50. Encapsulant layer 60 can
prevent photo-oxidation of the population of quantum dots in active
layer 30. Although FIG. 4 illustrates the deposit of both
encapsulant layer 60 and encapsulant layer 50, it is understood
that only encapsulant layer 50 or only encapsulant layer 60 can be
employed. Further, although encapsulant layers 60 and 50 are shown
as single layers, either or both can be applied as more than one
layer. Further, other layers can be disposed between or one any of
the active layer, encapsulant layer 50, and/or encapsulant layer
60.
[0048] As shown in FIG. 4, in certain embodiments, a solid state
lighting device comprises a reflector cup 70 housing light source
20 and any of active layer 30, encapsulant layer 50, and
encapsulant layer 60. Although reflector cup 70 is shown in many of
FIGS. 1-4, it is understood that the figures only illustrate
exemplary embodiments and other embodiments of the present
invention include a solid state lighting device without a reflector
cup. A reflector cup can cause the emitted light from the light
source to reflect upwards so that sufficient amounts reach the
population of quantum dots in the active layer and excite them.
[0049] The solid-state lighting devices of the present invention
comprise a light source which may be, for example, an LED chip, a
laser, white light, lamp or any other suitable combination thereof.
Regarding LED chips, different light-emitting chips produce
distinct colors where, the wavelength of the light emitted from the
chip is dependant on the material bandgap and hence the
semiconductor composition with which the light-emitting chip is
made. Typically, visible emitting LED chips are made from gallium
phosphide alloyed or doped with varying amounts of aluminum and
indium (AlInGaP) or gallium nitride alloyed with varying amounts of
indium (InGaN) to produce light emission wavelengths ranging from
.about.580 nm to .about.680 nm (amber through deep red) and
.about.380 nm to 520 nm (UV through blue-green) respectively.
Preferably, in solid state lighting devices of the present
invention, the LED chip is selected such that it emits light at an
energy that is capable of exciting the quantum dots present in the
active layer. Typically, quantum dot complexes may be excited by
wavelengths that are less than the emission wavelengths of the
quantum dot. Alternative LED chip compositions may be used that
emit light in different portions of the spectrum so long as the
peak emission wavelength of the chip is shorter than the peak
emission wavelength of the overlying quantum dots. For example,
lead sulfide (PbS) emitting at 1550 nm in the infrared portion of
the spectrum may be used in conjunction with GaAs, InGaAs, or other
infrared emitted LEDs so long as they emit at wavelengths shorter
than 1550 nm. In a preferred embodiment of the present invention,
the light emitted from the LED chip is between 440 nm to 480 nm. Of
course, other LED chips may be used including but not limited to
480-530 nm "green" emitting InGaN LEDs, 400-420 nm violet emitting
InGaN LEDs, 380 nm UV emitting and other LED chips.
[0050] The solid state lighting devices of the present invention
have several different applications. For example, the devices can
be incorporated into traffic signals, signage and display lighting,
architectural lighting, LCD display backlights used in mobile
phones and PDAs, larger flat panel LCD backlights and
projectors/projection TV, outdoor/landscape lighting luminaires,
interior illumination in the transportation sector (airplanes,
subways, ships, etc.), automobiles, and a number of other
apparatuses.
[0051] The present invention also provides methods of manufacturing
a solid state light emitting device comprising dispersing one or
more population of quantum dots in a polymer or silicone having
acrylate groups to form a first matrix material, depositing the
first matrix material either directly or indirectly on a light
source and cross-linking the acrylate groups in the first matrix
material to form a solid active layer. Acrylate side chains have
the general chemical formula of (CH.sub.2.dbd.CHCOO.sup.-), thus
including vinyl groups attached to a carbonyl carbon. Quantum dots
may be dispersed within a liquid monomer or oligomer of a polymer
or silicone acrylate, dispensed onto the light source, and
cross-linked to form a solid active layer. The quantum dots may be
dispersed in the first matrix material with or without a
solvent.
[0052] The cross-linking can be brought about, for example, by
chemical additives, UV radiation, electron beam or heat. Typical
cross-linking reactions can be the result of UV or thermal
initiated vinylic addition, Michael addition, epoxidation, or
condensation. Typical cross-linking reactions involve the reaction
of the vinyl groups present on the polyacrylate side chains. In a
Michael reaction, a cross-linking molecule reacts with the carbonyl
carbons of the acrylate side chains on adjacent polymer oligomers.
Nonlimiting examples of crosslinking agents used in a Michael
reaction are amines, diamine, oleyl amine, dodecyl amine,
aminopropylmethoxysilane, bis(3-aminopropyl)-tetramethyl
disiloxane, 3-aminopropyl dimethyl ethoxysilane,
3-aminopropylmethyl bis-(trimethyl siloxy) silane, and combinations
thereof. The cross-linking reaction may be facilitated by adding
thermal initiators and/or UV initiators followed by the application
of heat or UV irradiation, respectively. An exemplary thermal
initiator is azobisisobutyronitrile (AIBN) and an exemplary UV
initiator is 1 hydroxy cyclohexyl phenyl ketone. In preferred
embodiments, the curing dosage is approximately 0.6 Joule/cm.sup.2
when utilizing UV curing.
[0053] In certain embodiments, 20% methyl hexahydrophthalic
anhydride (MHHPA) is added to the first matrix material of the
active layer. It has been found that the addition of 20% MHHPA to
the polymer or silicone acrylate matrix facilitates the dispersal
of quantum dots into the first matrix material and further improves
operational longevity. It has also been found that epoxy may be
added to the first matrix material applied to the light source and
cured without the degradation that is observed when quantum dots
are directly dispersed into epoxy.
EXAMPLES
Example 1
Various Active Layer Locations in Device
[0054] FIG. 5 illustrates the relationship between the distance
between the light source and the active layer and the intensity of
the light emitted by a solid state lighting device according to an
embodiment of the present invention. In this example, three solid
state lighting devices were fabricated with reference to the solid
state lighting device illustrated in FIG. 4, the devices having
different volumes of the second matrix material forming the first
encapsulant layers (50) deposited on the active layers (30),
thereby providing different distances between the active layers
(30) and the light sources (20).
[0055] Here, the devices were fabricated on low power SMD-type LED
chips, such as those LED chips produced by Knowledge-On Inc. The
Knowledge-On LED chip has the form factors with 2.4 mm in diameter
and about 1 mm in depth and surrounded by a white plastic cup.
OP-54 (Dymax), a UV curable polyurethane acrylate, was used as the
second matrix material (51) forming the first encapsulant layers
(50) of each device. To form the first encapsulant layers (50),
three different amounts of OP-54 were first deposited on the LED
chips of the three devices in respective volumes of 0.0, 1.4, and
2.1 .mu.l, where each 1 .mu.l can make thickness in the range of
0.1-1 mm, which is dependant on the LED chip size, type and form
factors. The OP-54 was then cured under ultraviolet radiation to
form the first encapsulant layers.
[0056] To prepare the active layer (30) of each device, red light
emitting CdSe quantum dot complexes were dispersed in a mixture of
OP-54 (the first matrix material) and 20% methyl hexahydrophthalic
anhydride (MHHPA), with a quantum dot complex concentration of 10
mg/ml. The same amount of OP-54 containing CdSe quantum dot
complexes was disposed on the first encapsulant layers (50) for all
three devices and cured under ultraviolet radiation to form the
active layers (30).
[0057] To form the second encapsulant layers (60), the same volume
1.2 .mu.l of OP-54 (the third matrix material (61)) was deposited
on the active layers (30) of the three devices to form a dome shape
and cured under ultraviolet radiation. This resulted in the three
solid state lighting devices.
[0058] The intensity of the light emitted by the three devices was
tested as follows. All devices were operated at 20 mA with voltage
about 3.2V and quantum dot emissions measured. FIG. 5 shows the
emission peak for the LED chip at around 460 nm and the emission
peak for the quantum dots at around 625 nm. In the figure, the
relative intensity of the light emitted by the quantum dots
decreased with increased active layer distance from the LED chip.
As demonstrated here, the active layer absorbs more photons from
the light source when the active layer is closer to the light
source.
Example 2
Green Light-Emitting Quantum Dots with Blue Light Source
[0059] FIG. 6 illustrates the spectral response of a solid state
lighting device according to an embodiment of the present invention
when emitting green light from the quantum dots and having a blue
light source. In this example, a green LED was fabricated with
reference to the device of FIG. 4. A UV-curable silicone matrix
material was used for the active layer (30), the first encapsulant
layer (50), and the second encapsulant layer (60). The delivered
volumes of matrix material for the layers were 2.0, 1.5, and 8
.mu.l, respectively. To form the active layer, a mixture of 20%
methyl hexahydrophthalic anhydride (MHHPA) and 80% silicone matrix
material was embedded with 10 mg/ml of CdSe quantum dots. The
spectral power density of the green LED was then measured.
[0060] Quantum yield is defined as a fraction of the number of
quantum dot complex photons coming out of the number of absorbed
photons, which is measured with very dilute concentration of
quantum dots (.about.0.01 mg/ml) in an organic solvent. Conversion
efficiency is also defined as the quantum yield with realistic
concentration of quantum dots (>0.1 mg/ml) in the active layer
when placed on the solid-state source. In this example, the quantum
yield of the green LED was measured at about 74% and the conversion
efficiency was measured at about 71%.
[0061] Typically, the efficiency significantly decreases by between
10 and 30% when the high concentration quantum dots are solvated in
a matrix material and placed on an LED chip. However, as shown
above, such was not the case with the green LED, where the
conversion efficiency was very close to the original quantum yield
without efficiency loss, even for the very high concentration 10
mg/ml. Thus, the LED of this example provides for a solid-state
lighting device in which the conversion efficiency is substantially
maintained upon the introduction of quantum dot complexes into the
first matrix material. Preferably, the conversion efficiency after
incorporation into the first matrix material is 80% of the
efficiency of the underlying quantum dot complexes, more preferably
90%, and most preferably 95%.
[0062] As shown in FIG. 6, the green LED of the present example was
found to have a very high efficacy 22.4 lm/W compared to the blue
source LED chip efficacy 5.3 lm/W. The reasons for the high
efficacy of the green LED was the high quantum dot conversion
efficiency of the active layer and high human eye sensitivity to
green. Inherently, traditional semiconductor LEDs cannot achieve
high efficiency in the green to orange region (540-590 nm), where
the efficiency of these LEDs has been on the order of 4-9 lm/W. For
the green to orange color region, more than 60% conversion
efficiency of the LED is viable in the LED light industry.
Example 3
White Light-Emitting Quantum Dots with Blue Light Source
[0063] FIG. 7 illustrates the spectral response of a solid state
lighting device according to an embodiment of the present invention
when emitting white light from quantum dots and having a blue light
source. Here, a white LED was fabricated with reference to the
device of FIG. 4. A UV curable resin OP-54 was used for the active
layer (30), the first encapsulant layer (50) and the second
encapsulant layer (60). For the active layer (30), two CdSe quantum
dot complexes, green and red light-emitting quantum dots, were
mixed in toluene solvent with concentrations of 10 mg/ml for green
and 1.3 mg/ml for red light-emitting quantum dot complex. 0.45
.mu.l of the quantum dot solution in toluene was directly delivered
on a first matrix material (35) of the active layer (30) without
solvating the quantum dots in the first matrix material (35). After
deposition of the active layer (30) on the first encapsulant layer
(50), the device was dried in a vacuum oven to evaporate the
organic solvent. The drying time is about 1 hour at 70 degrees
Celsius. Then, the second encapsulant layer (60) was formed and
UV-cured on top of the active layer (30). The emissions of the
resulting white LED was measured.
[0064] As shown in FIG. 7, the blue light source emitted at a
wavelength of about 460 nm, the green quantum dots emitted at about
555 nm, and the red quantum dots emitted at about 615 nm. The
resulting white light corresponds to color coordinates (0.32, 0.32)
of CIE 1931. The white LED performed at 15 lm/W in efficacy, 47% in
quantum dot conversion efficiency, 6100 K in correlated color
temperature, and 88 in color rendering index.
Example 4
Infrared Emitting Quantum Dots with Blue Light Source
[0065] FIG. 8 shows the spectral response of a solid state lighting
device according to an embodiment of the present invention when
emitting infrared (IR) light from quantum dots and having a blue
light source. In this example, an IR emitting LED was fabricated
with reference to the device of FIG. 4. The active layer (30) was
formed by solvating the concentration 10 mg/ml of PbS quantum dots
in a mixture of 20% MHHPA and 80% silicone. The PbS quantum dot
active layer (30) was placed between two encapsulant layers (50 and
60). Each encapsulant layer was silicone. The delivered volumes of
the active (30), first encapsulant (50), and second encapsulant
(60) layers were 2.0, 1.5, and 8 .mu.l, respectively.
[0066] As shown in FIG. 8, the blue band is the remnant source
emission, which was not absorbed by the quantum dots. The PbS
quantum dots' emission band is located at 910 nm with a very broad
spectrum. The conversion efficiency of the PbS quantum dot device
was measured to be 82%.
Example 5
Multiple Active Layers with Red and Green Quantum Dots
[0067] FIG. 9 illustrates the spectral response of a solid state
lighting device according to an embodiment of the present invention
when emitting red and yellow light from multiple active layers. In
this example, the multiple active layer device was fabricated as
described in Example 1. The first encapsulant layer (50) and the
second encapsulant layer (60) were the same as in Example 1, but
instead of a single active layer, there were two active layers. The
first active layer comprised red light-emitting quantum dots and
the second active layer comprised green light-emitting quantum
dots. Specifically, 0.3 .mu.l of 2.5 mg/ml red quantum dots in
OP-54 were deposited on the first encapsulant layer (50) and cured
under UV irradiation. On top of the red quantum dot layer, 0.5
.mu.l of OP-54 spacer film was made to space apart the active
layers. Then, 0.5 .mu.l of green emitting quantum dots (20 mg/ml in
toluene solvent) was deposited on the spacer layer. To remove the
remnant solvent in the second active layer, the deposited material
was dried in a vacuum oven at 80 degrees Celsius for 1 hour.
[0068] As shown in FIG. 9, the blue light source emitted at a
wavelength of about 460 nm, the green quantum dots emitted at about
555 nm, and the red quantum dots emitted at about 625 nm. Each
emission band and intensity can be adjusted by concentration and
volume of the red and green emitting quantum dots.
Example 6
Device Stability
[0069] One problem associated with the use of quantum dot complexes
as a phosphor is that such devices typically degrade quickly over
time. Embodiments of the present invention overcome this drawback
through the use of structured layers and the use of an encapsulant
layer (typically urethane acrylate) that reduces oxygen
permeability.
[0070] In this example, a solid state lighting device was
fabricated as described in Example 1. Cree 1411 SMD blue LED chips
emitting at 460 nm were used as the light source. The structures
and materials for the encapsulant and active layers were the same
as in Example 1. The active layer was formed by doping the first
matrix material with green and red emitting quantum dots at
concentrations of 6.8 and 1.7 mg/ml, respectively, to produce a
"warm" ( T.about.3000K) white light. In another device, the doping
for a "cooler" white (T.about.6000K) was 4.5 and 0.5 mg/ml for
green and red, respectively. 1.7 .mu.l of a second matrix material
(51) was deposited on the blue LED chips and cured by UV
irradiation for 20 seconds to form the first encapsulant layer
(50). Volumes of 1-1.7 .mu.l of the doped first matrix material
(35) were then deposited on the first encapsulant layer (50) and
cured for 60 seconds to form the active layer (30). 1.7 .mu.l of a
third matrix material (61) was then deposited on the active layer
(30) and cured for 20 seconds to form the second encapsulant layer
(60).
[0071] Device intensities were tested over time and under a
continuous DC current of 30 mA. The stability of the resulting
white LEDs was monitored for 4000 hours, as shown in FIG. 10. The
degradation rate of the underlying blue light source at 4000 hours
was approximately 0.014%/hour; whereas, the degradation rates of
the quantum dots were 0.0096%/hour for the green quantum dots and
0.0075%/hour for the red quantum dots.
Example 7
One Layered Device with Lamp-Type Light Source
[0072] A solid state lighting device according to an embodiment of
the present invention was fabricated having one layer. A lamp-type
LED chip, such as supplied by Optosupply, was used as the light
source, with emissions at about 460 nm. The small cup size of the
lamp-type LED chip made it desirable to deposit a single layer
thereon. In this example, 0.26 .mu.L of OP-54 with quantum dots
dispersed therein was deposited on the LED chip and cured by UV
irradiation for 30 seconds. The device was then immersed in a 5 mm
epoxy filled cap mold and heated at 110 Celsius for 8 hours.
Example 8
Scattering Elements to Improve Color Uniformity
[0073] In some conventional light emitting devices, e.g., lamp and
surface mounted device (SMD) types, the light source and the
quantum dot emissions are not uniform, such that the color observed
changes depending on the viewing angle. The present invention
solves this problem by adding non-absorbing light scattering
dielectric particles.
[0074] In this example, an SMD was fabricated as follows. In the
active layer, titania (TiO.sub.2) nanoparticles, averaging 5 nm in
diameter, were added at an optimal 1% by weight amount to OP-54
(the first matrix material) dispersed with red emitting quantum
dots. Titania was not added to the two encapsulant layers of OP-54.
As a result, the angular uniformity of the color emitted by the
solid state lighting device improved greatly. The spectrum of the
emitted light is shown in FIG. 11.
[0075] A lamp-type device was also fabricated as described in
Example 7. The lamp-type device comprised one layer of quantum
dots, matrix material, and titania. The device was cured under UV
radiation and then encapsulated in epoxy as described in Example
7.
Example 9
Red Light-Emitting Quantum Dots with Green Light Source
[0076] In the previous examples, the underlying light source was a
blue light source. In this example, a green light source was used.
A green 1411 SMD LED chip, such as supplied by Optosupply, with
emissions at about 525 nm was used as the light source. Red
emitting quantum dots with emissions at about 602 nm were used in
the active layer.
[0077] Four solid state lighting devices were fabricated as
follows. A first device was fabricated of the green light source
itself with UV-cured matrix material on it. The second, third, and
fourth devices were fabricated according to embodiments of the
present invention. The second, third, and fourth devices had three
layers, in which the first and second encapsulant layers were
formed of OP-54 and the active layers were formed of 1.75, 5.25,
and 10.5 micrograms, respectively, with red emitting quantum dots
dispersed in OP-54. The emitted colors were measured.
[0078] FIG. 12 shows the measured coordinates of the emitted colors
for the four devices on the CIE 1931 diagram. The results show an
increase in the area of the CIE 1931 diagram that is accessible
with an increase in the amounts of quantum dots in the device.
Example 10
Quantum Dots with UV light source
[0079] Colors that are observed from a light emitting device having
a blue light source are typically a mixture of the colors emitted
by the quantum dots and the blue light source, unless enough
quantum dots are dispersed in the active layer to saturate the
color. However, while saturation is one way to produce a solid
state lighting device in which only the color emitted by the
quantum dots is observed, the efficiency of the device may be
lowered somewhat due to reabsorption effects of the quantum dots.
An alternative to saturation provided by the solid state lighting
device of the present invention is to use an ultraviolet light
source.
[0080] In this example, an ultraviolet LED chip, having emission at
about 407 nm and of a 1411 SMD type, was used as the light source.
Green, yellow, and red emitting quantum dots, having emissions at
about, 536, 567, and 602 nm, respectively, were used in the active
layers.
[0081] Four solid state lighting devices were fabricated as
follows. A first device was fabricated of the UV light source
itself with UV-cured matrix material on it. The second, third, and
fourth devices were fabricated according to embodiments of the
present invention. The second, third, and fourth devices had three
layers, in which the first and second encapsulant layers were
formed of OP-54 and the active layers were formed of quantum dots
dispersed in OP-54. The second device had 10 micrograms of green
emitting quantum dots in the active layer. The third device had 7
micrograms of yellow emitting quantum dots in the active layer. The
fourth device had 3.5 micrograms of red quantum dots in the active
layer. The emitted colors were measured.
[0082] FIG. 13 shows the measured coordinates of the emitted colors
for the four devices on the CIE 1931 diagram. Using an ultraviolet
LED chip provided two advantages. First, the quantum dots had
higher absorption coefficients in the UV range, i.e., they absorb
more of the UV light. Second, the ultraviolet light source
contributed very little to the actual color of the device, such
that less quantum dots were required to get the true color emitted
by the quantum dots.
Example 11
Thermally Cured Red Light-Emitting Device
[0083] FIG. 14 shows a spectral response of a solid state lighting
device of the present invention that was thermally cured. In this
example, a thermally cured lamp-type LED was constructed using
mixtures of epoxy and OP-54. To prepare the active layer, OP-54 was
dispersed with red emitting quantum dots, having emission at about
602 nm, and mixed with epoxy in a ratio of 1:6, OP-54: epoxy by
volume, resulting in a quantum dot concentration of about 1.43
mg/ml. The resulting mixture was deposited on an LED chip at a
volume of 0.5 .mu.l and then thermally cured for 30 minutes at 120
degrees Celsius. After cooling, a 5 mm epoxy cap was added as
described in Example 7. The spectral response was then
measured.
Example 12
Thermally Cured Warm White Light-Emitting Device
[0084] A thermal curing process was also used to construct a solid
state lighting device according to an embodiment of the present
invention that emits warm white light, having a correlated color
temperature (CCT) of about 3000K and a high color rendering index
(CRI) of about 94. An underlying chip, a Cree 1411 SMD with an
emission peak at about 460 nm, was used as the light source. Red
emitting quantum dots, having emission at about 610 nm, in
conjunction with cesium yttrium aluminum garnet powder (Ce:YAG)
from Osram were used in the active layer.
[0085] To prepare the active layer, OP-54 was dispersed with the
red emitting quantum dots and the Ce:YAG powder and mixed with
epoxy in a ratio of 1:10, OP-54:epoxy, resulting in a quantum dot
concentration of about 0.45 mg/ml and a Ce:YAG concentration of
about 200 mg/ml. A 1.5 .mu.l layer of epoxy was deposited on the
chip and cured at 120 degrees Celsius for one hour. A 1.5 .mu.l of
the active layer was deposited on top of the epoxy layer and cured
for one hour at 120 degrees Celsius. The spectrum of the resulting
warm white, high CRI LED is shown in FIG. 15.
[0086] The foregoing description and examples have been set forth
merely to illustrate the invention and are not intended as being
limiting. Each of the disclosed aspects and embodiments of the
present invention may be considered individually or in combination
with other aspects, embodiments, and variations of the invention.
Further, while certain features of embodiments of the present
invention may be shown in only certain figures, such features can
be incorporated into other embodiments shown in other figures while
remaining within the scope of the present invention. In addition,
unless otherwise specified, none of the steps of the methods of the
present invention are confined to any particular order of
performance. Modifications of the disclosed embodiments
incorporating the spirit and substance of the invention may occur
to persons skilled in the art and such modifications are within the
scope of the present invention. Moreover, it is appreciated, that
although a number of problems and deficiencies may be identified
herein, each embodiment may not solve each problem identified in
the prior art. Additionally, to the extent a problem identified in
the prior art or an advantage of the present invention is cured,
solved, or lessened by the claimed invention, the solution to such
problems or the advantage identified should not be read into the
claimed invention. Furthermore, all references cited herein are
incorporated by reference in their entirety.
* * * * *